Good luck with the skydive! I've done some and it's great fun

If you just remember to keep your chin UP then you can breathe normally. If you look straight down into the airflow then you will have more difficulty breathing! They will tell you all that and a lot more that you need to know at your briefing before the jump though. Are you having the jump filmed? If you get the option (most places will offer it) then I would definitely recommend you getting it filmed. It will go so quickly it'll be over before you know it!
As long as you remember the instructions your tandem instructor gives you, you will be absolutely fine. Even if you DON'T remember to do anything he has told you, you will be fine. They are very highly trained so they can compensate for people who ignore everything they were told. So if they have someone strapped to their front who flails their limbs about all over the place, they can still make adjustments with their own body to make your freefall safe and smooth

They train with huge weighted bags strapped to their front in freefall, so they are used to carrying the additional weight of an additional person.
It will seem very noisy in freefall just because of the speed at which you are falling, also your face feels really funny as the air pushes your cheeks up, so if you do have pictures or video/dvd you will really laugh at the way you look

Once the canopy opens it's like instant silence! A very strange sensation the first time you do it but really peaceful. You'll probably then spend 3 to 5 minutes under canopy before you land and 99% of the time your landing will be so smooth that you hardly feel it (the other 1% of the time you get a bump on the bum that's all, just like sitting down heavily in a chair)
Hope you really enjoy it! I am sure you will. If you are at all worried remember these instructors do this hundreds of times a year with people of all different shapes and sizes and all different levels of fear. I have so much confidence in their training that I would go with a new tandem instructor as his/her first passenger with no doubts in my mind that it was safe to do so. I hope that will give you confidence
I used to pack the parachutes for tandems, so I also have a lot of confidence in the equipment used, how it is packed, and the checks it all undergoes. I've never yet met anyone who regretted doing a tandem and I've seen thousands, so that says something about it!
If you have any questions feel free to ask. I'll be here again later this evening.
Have fun!
Mocha